1. Field of the Invention:
This invention relates to an insulated-gate semicustom integrated circuit, and more particularly to protection of internal circuits connected to input or output pads of the integrated circuit against electrostatic breakdown.
2. Description of the Related Art:
In general, insulated-gate, e.g. MOS (metal-oxide semiconductor) semicustom integrated circuits are provided with protection circuits in macrocells of the input/output section in order to protect internal circuits from electrostatic breakdown. A prior art protection circuit is arranged as shown in FIG. 1. That is, a diode 3 is connected between a signal line 2 connected to an input/output pad 1 and a high-potential supply voltage V.sub.DD node, while a diode 4 is connected between signal line 2 and a low-potential supply voltage V.sub.SS node (usually ground potential node). Further, a protective resistor 5 is provided between signal line 2 and internal circuit 7 to be protected.
However, the conventional protection circuit composed of diodes 3 and 4, and resistor 5 has not yet achieved a satisfactory protection strength against electrostatic breakdown. To obtain a satisfactory strength of protection against electrostatic breakdown it is required that the constituent components of the protection circuit be made large in size. In this case, however, the macrocell is also required to be made large in size, thus providing a great difficulty in making integrated circuits small.